In recent years, due to the development of increasingly miniaturized silicon devices, a device forming region is required not to have crystal defects which may cause an increase in leak current and a decrease in lifetime of a carrier. To meet the requirement, an epitaxial wafer, in which an epitaxial layer is grown on a silicon wafer, is produced, and the surface epitaxial layer is used as the device forming region.
Meanwhile, one problem in the process of manufacturing such a silicon device may be contamination of heavy metals into the wafer. For example, the wafer contaminated with heavy metals, such as cobalt, copper, and nickel, will cause harmful effects on the device characteristics, including poor pose time, poor retention, poor junction leak, and insulation breakage of an oxide film. In view of the above, a gettering method is typically adopted to suppress diffusion of the heavy metals into the device forming region.
Examples of a gettering method include an intrinsic gettering method (IG method) of precipitating oxygen in the wafer to use the formed oxide precipitates as a gettering site, and an extrinsic gettering method (EG method) of applying mechanical strain to a back surface of the wafer by a sand blast process or the like or of forming, as a gettering site, a polycrystalline silicon film or the like on the back surface.
However, due to the lowered temperature at the device formation process and the increased diameter of the silicon wafer, the problem that the silicon wafer, and thus the epitaxial wafer are not imparted with a sufficient gettering capability has occurred. That is to say, the lowered temperature at the formation process has made it difficult to form oxide precipitates in the wafer. Furthermore, in cases of the silicon wafer having a diameter of 300 mm or more, typically, not only the main surface but also the back surface is subject to mirror-polishing processing, and therefore, mechanical strain cannot be applied to the back surface of the wafer, and a polycrystalline silicon film cannot be formed thereon. Thus, at the present, imparting a gettering capability to the wafer is difficult.
Under the above background, Patent Literature 1 proposes, as a method of imparting a gettering capability to an epitaxial wafer, the technology of producing an epitaxial wafer having an excellent gettering capability by implanting carbon ions into a surface of a silicon wafer, producing a silicon wafer in which a gettering layer, including a region (hereinafter, called the “high concentration carbon region”) containing high concentration carbon, is formed on a surface portion of the silicon wafer, and subsequently forming an epitaxial layer on the surface of the produced silicon wafer.
However, when contaminating metals attach to the surface of the wafer at the time of forming the epitaxial layer on the silicon wafer or forming a device element on the device forming region, the contaminating metals are unable to detach from the device forming region due to the lowered temperature at the device forming process, thereby possibly failing to be captured by the gettering sites located deep beneath the surface of the wafer.
Moreover, to form the gettering layer by implanting carbon ions at a high concentration into the location deep beneath the surface of the wafer, acceleration voltage of the carbon ions needs to be increased. This may lead to degradation of crystallinity on the surface of the wafer, resulting in the problem of defects of the epitaxial layer grown thereon.
As a method for solving the problems, Patent Literature 2 describes the technology of manufacturing an epitaxial wafer that solves the problem of degradation of crystallinity on the surface of the wafer and that also has an improved gettering capability, by forming a modifying layer as the gettering layer including the high concentration carbon region by irradiating the wafer with ions of cluster of a plurality of atoms or molecules into a shallow location very close to the surface of the silicon wafer.